Brachytherapy is a general term covering medical treatment which involves placement of a radioactive source near a diseased tissue and may involve the temporary or permanent implantation or insertion of a radioactive source into the body of a patient. The radioactive source is thereby located in proximity to the area of the body which is being treated. This has the advantage that a high dose of radiation may be delivered to the treatment site with relatively low dosages of radiation to surrounding or intervening healthy tissue.
Brachytherapy has been proposed for use in the treatment of a variety of conditions, including arthritis and cancer, for example breast, brain, liver and ovarian cancer and especially prostate cancer in men (see for example J. C. Blasko et al., The Urological Clinics of North America, 23, 633-650 (1996), and H. Ragde et al., Cancer, 80, 442-453 (1997)). Prostate cancer is the most common form of malignancy in men in the USA, with more than 44,000 deaths in 1995 alone. Treatment may involve the temporary implantation of a radioactive source for a calculated period, followed by its removal. Alternatively, the radioactive source may be permanently implanted in the patient and left to decay to an inert state over a predictable time. The use of temporary or permanent implantation depends on the isotope selected and the duration and intensity of treatment required.
Permanent implants for prostate treatment comprise radioisotopes with relatively short half-lives and lower energies relative to temporary sources. Examples of permanently implantable sources include iodine-125 or palladium-103 as the radioisotope. The radioisotope is generally encapsulated in a titanium casing to form a xe2x80x9cseedxe2x80x9d which is then implanted. Temporary implants for the treatment of prostate cancer may involve iridium-192 as the radioisotope.
Recently, brachytherapy has also been proposed for the treatment of restenosis (for reviews see R. Waksman, Vascular Radiotherapy Monitor, 1998, 1, 10-18, and MedPro Month, January 1998, pages 26-32). Restenosis is a re-narrowing of the blood vessels after initial treatment of coronary artery disease.
Coronary artery disease is a condition resulting from the narrowing or blockage of the coronary arteries, known as stenosis, which can be due to many factors including the formation of atherosclerotic plaques within the arteries. Such blockages or narrowing may be treated by mechanical removal of the plaque or by insertion of stents to hold the artery open. One of the most common forms of treatment is percutaneous transluminal coronary angioplasty (PTCA)xe2x80x94also known as balloon angioplasty. At present, over half a million PTCA procedures are performed annually in the USA alone. In PTCA, a catheter having an inflatable balloon at its distal end is inserted into the coronary artery and positioned at the site of the blockage or narrowing. The balloon is then inflated which leads to flattening of the plaque against the artery wall and stretching of the artery wall, resulting in enlargement of the intraluminal passage way and hence increased blood flow.
PTCA has a high initial success rate but 30-50% of patients present themselves with stenotic recurrence of the disease, i.e. restenosis, within 6 months. One treatment for restenosis which has been proposed is the use of intraluminal radiation therapy. Various isotopes including iridium-192, strontium-90, yttrium-90, phosphorus-32, rhenium-186 and rhenium-188 have been proposed for use in treating restenosis.
Conventional radioactive sources for use in brachytherapy include so-called seeds, which are sealed containers, for example of titanium or stainless steel, containing a radioisotope within a sealed chamber but permitting radiation to exit through the container/chamber walls (U.S. Pat. Nos. 4,323,055 and 3,351,049). Such seeds are only suitable for use with radioisotopes which emit radiation which can penetrate the chamber/container walls. Therefore, such seeds are generally used with radioisotopes which emit xcex3-radiation or low-energy X-rays, rather than with xcex2-emitting radioisotopes.
Brachytherapy seeds comprising a coating of radioactive silver iodide on a silver wire encapsulated inside a titanium container are known in the art (U.S. Pat. No. 4,323,055). Such seeds are formed by first chloriding or bromiding the silver to form a layer of silver chloride or bromide, and then replacing the chloride or bromide ions with radioactive iodide ions (I-125) by ion exchange. Such seeds are available commercially from Medi-Physics, Inc., under the Trade Name I-125 Seed(copyright) Model No. 6711 or OncoSeed(trademark) Iodine-125 seeds (Nycomed Amersham).
Other conventional brachytherapy seeds comprise titanium containers encapsulating ion exchange resin beads onto which a radioactive ion, for example I-125, has been absorbed (U.S. Pat. No. 3,351,049). The immobilisation of a radioactive powder within a polymeric matrix has also been proposed (WO97/19706).
The processes disclosed in U.S. Pat. No. 4,323,055 for the production of I-125 containing seeds involve a number of separate steps. We believe a more efficient and rapid method for the production of radioactive sources comprising insoluble salts, especially silver salts, is desirable from a manufacturing viewpoint.
According to one aspect of the invention there is therefore provided a method for the immobilisation of one or more radioisotopes on the surface of a metal substrate, said method comprising treating the substrate with an oxidising agent to produce metal cations, in the presence of a source of a radioactive anion containing one or more radioisotopes, which anion forms an insoluble salt with said metal cations. Preferably, the radioactive anion will be present in solution or in a dispersion. Preferably, a binding agent will also be present. The products of the method of the invention are radioactive substrates.
Any metal which can form an insoluble salt with a radioactive anion on oxidation may be used as the metal substrate in the method of the invention. Suitable metals include silver, copper, lead, zinc, palladium, thallium, cadmium, lanthanum and gold. Preferably, the metal substrate is silver. The substrate may be made of solid metal or a suitable material plated with a layer of metal, for example silver, zinc, palladium or thallium. Suitable materials for plating include other metals, for example gold, copper or iron, and plastics, for example polypropylene, polystyrene, nylon, delrin, Kevlar(trademark), and any other plastic or composite which can form a solid rod for plating with the metal of interest. Suitable plating methods are known in the art and include chemical deposition, sputtering and ion plating techniques.
The substrate should be of a suitable size and dimensions for incorporation into a source, for example a seed, for use in brachytherapy. Conventional seeds for use in the treatment of prostate cancer, for example, are typically substantially cylindrical in shape and approximately 4.5 mm long with a diameter of approximately 0.8 mm, such that they may be delivered to the treatment site using a hypodermic needle. For use in the treatment of restenosis, a source should be of suitable dimensions to be inserted inside a coronary artery, for example with a length of about 10 mm and a diameter of about 1 mm, preferably a length of about 5 mm and a diameter of about 0.8 mm, and most preferably with a length of about 3 mm and a diameter of about 0.6 mm. Sources for use in the treatment of restenosis are typically delivered to the treatment site using conventional catheter methodology.
Preferably, the substrate is of a suitable size and dimensions to fit inside a conventional seed container, such as those disclosed in U.S. Pat. No. 4,323,055 which is hereby incorporated by reference. Preferred seed containers are those made of titanium, titanium alloy or stainless steel. Preferably, the substrate will be substantially cylindrical in shape, for example in the form of a rod or wire. Suitable dimensions are about 3 mm long and about 0.10 mm to 0.70 mm in diameter, preferably about 0.5 mm in diameter.
Alternatively, the radioactive substrates may be incorporated into a polymer or ceramic matrix. Suitable polymer matrices include those disclosed in WO97/19706 which is hereby incorporated by reference. If the radioactive anion comprises a xcex2-emitter, the radioactive substrate should not be encapsulated in a metal container as such containers would absorb the xcex2-particles emitted and prevent them from reaching the treatment site.
If the metal substrate comprises silver or another X-ray opaque metal such as gold, copper or iron, there is the added advantage that sources comprising the radioactive substrate will be detectable by X-ray when inserted or implanted into a patient. Preferably, the substrate is shaped such that its orientation can also be determined by X-ray imaging. If the substrate comprises an X-ray transparent material plated with a metal such as silver, the radiopaque metal thickness is preferably greater than about 0.050 mm to ensure X-ray visualisation.
The radioactive anions for use in the method of the invention may be simple anions such as 125Ixe2x88x92 or 35S2xe2x88x92, or complex anions such as 32PO43xe2x88x92, 35SO42xe2x88x92 or 51CrO42xe2x88x92. If the anion is a complex anion, it may comprise one or more radioisotopes, preferably one, two or three radioisotopes. More than one type of radioactive anion may be used together in the method of the invention, for example 125Ixe2x88x92 together with 35SO42xe2x88x92. The choice of anion(s) and radioisotopes will depend in part on the intended use of the resulting brachytherapy source and the type of radiation which the sources should emit. For example, the radioactive anion may emit xcex3-radiation or low-energy X-rays, or it may be a xcex2-emitter.
A radioactive anion which forms an insoluble salt with the metal of the metal surface of the substrate should be used. Preferred radioactive anions include those comprising 125Ixe2x88x92, 35S, 32P or 33P. Possible anions include 125Ixe2x88x92, 131Ixe2x88x92, 123Ixe2x88x92, 35S2xe2x88x92, 35SO42xe2x88x92, 35SO32xe2x88x92, 125IO3xe2x88x92, 131Ixe2x88x92, 123IO3xe2x88x92, 51CrO42xe2x88x92, 32PO43xe2x88x92, H32PO42xe2x88x92 and H232PO4xe2x88x92. For the purposes of the invention, a salt is considered to be insoluble if its solubility product constant is lower than about 1xc3x9710xe2x88x925, preferably less than about 1xc3x9710xe2x88x922 and most preferably less than 1xc3x9710xe2x88x9216. For a sparingly soluble salt MxAy in contact with its saturated solution, the solubility product is given by Ksp=[My+]x[Axxe2x88x92]y.
Concentrations are normally given in moles/liter at 298xc2x0 K. For example, if the metal is silver, suitable anions include those shown in the Table 1 below. Values are taken from the Handbook of Chemistry and Physics, 74th Edition, 1993-4, Section 8, page 49.
Other possible anions include complex anions derived from pyrophosphoric acid, such as H3P2O7xe2x88x92, wherein one or more of H, P or O comprise a radioisotope. Suitable anions derived from pyrophosphoric acid are disclosed in WO97/49335 which is hereby incorporated by reference. When the metal surface is silver, a preferred anion is 125I-iodide.
One advantage of the method of the invention is that it is a xe2x80x9cone-stepxe2x80x9d chemical reaction. The prior art chemical processes comprise two or more steps, which lead to longer preparation times, greater variability of iodide distribution and greater costs. The method of the invention is also readily applicable to substrates of a variety of different geometric shapes, for example spheres or rods. The method can be applied to a single substrate or to a plurality of substrates wherein the number of substrates can range from 2 to about 100,000 or more, for example in a batchwise process. Treatment of a metal substrate with an oxidizing agent in the presence of a radioactive anion in a one-pot reaction leads to immobilization of the anion on the substrate in situ. Preferably, both the oxidizing agent and the radioactive anion are used as solutions in the same solvent, for example in aqueous solution. Alternatively, a solid oxidising agent may be added to a reaction mixture comprising the metal substrate and a solution or dispersion of a radioactive anion.
The source of the radioactive anion may be present in solution in a suitable solvent. Alternatively, it may be present in the form of a dispersion or precipitate in a suitable liquid phase. For example, if the radioactive anion is iodide, it may be present as a dispersion of silver iodide, or a dispersion of any insoluble metal iodide salt which displays greater solubility than that of the insoluble radioactive salt to be formed using the method. For example, if the substrate is silver and the radioactive anion is iodide, the iodide source may be a dispersion of an iodide salt of copper, lead, palladium or thallium. Table 2 below shows the solubility of some iodide salts. Copper, lead, palladium and thallium iodide are all more soluble than silver iodide, and so could be used as an iodide source in the immobilisation of iodide ions on a silver substrate using the method of the invention. Gold iodide is less soluble than silver iodide and hence would not be a suitable iodide source in such a method using a silver substrate.
On oxidation of a silver substrate, the iodide should transfer from the dispersion to the surface of the substrate.
Alternatively, the source of radioactive iodide ions may be present in the form of an organic iodide-containing compound, for example an alkyl iodide such as 2-iodoethanol, ethyl iodide, iodobutylacetate, iodobutyric acid, 3-iodopropanol, epiiodohydrin, glyceryl iodide, an activated iodomethylcarbonyl compound such as iodoacetamide or iodoacetic acid, an iodinated lachrymator such as iodinated acetone or 1-iodo-2-(trimethylsilyl)acetylene, or an iodosilicon compound such as iodotrimethyl silane, each of which may degrade over the course of the immobilisation reaction to generate iodide ions in the solution phase.
Radioactive iodide ions may also be present in the form of a complex with a suitable complexing agent, for example starch, amylose, amylopectin, or another complex carbohydrate, which will gradually release iodine, and thence iodide ions, in the presence of hydroxide ions, into the solution phase over the course of the immobilisation reaction. Complexes of other radioactive anions may also be used in the method of the invention.
Alternatively, the radioactive anion source may be a suitable ion exchange resin bead with radioactive anions adsorbed thereon. Any ion exchange resin which can act as a reservoir for the radioactive anions may be used (for example, Pbxe2x88x922exe2x88x92xe2x86x92Pb2+, Resin-SO42xe2x88x92xe2x86x92Resin+SO42xe2x88x92, Pb2+xe2x86x92SO42xe2x88x92xe2x86x92PbSO4).
Suitable oxidizing agents are known in the art, including those disclosed in U.S. Pat. No. 4,323,055 which is herein incorporated by reference. They include sodium chlorite (NaClO2), sodium chlorate (NaClO3), sodium chromate (Na2CrO4), hydrogen peroxide (H2O2), potassium dichromate (K2Cr2O7), potassium permanganate (KMnO4), and potassium ferricyanide (K3Fe(CN)6). For the immobilisation of iodide ions on silver metal, a preferred oxidizing agent is potassium ferricyanide.
The oxidizing agent and the metal should be chosen such that the oxidizing agent can oxidize the metal surface of the substrate under the reaction conditions. The metal cations thus formed at the surface of the substrate should combine with the radioactive anions in solution to form a layer of an insoluble salt on the surface of the substrate, thus immobilizing the anions on the surface of the substrate. Immobilization of a radioactive anion on the metal surface of a substrate provides a radioactive metal substrate.
Whether or not a particular oxidising agent is suitable for use in the method of the invention with a particular metal and a particular radioactive anion can be predicted by reference to the standard electrode potentials of the relevant half-reactions. If the sum of the standard electrode potentials for the oxidation half-reaction and the reduction half-reaction is positive, then in the absence of inhibiting kinetic effects, reaction should occur spontaneously. Tables of standard electrode potentials are readily available, for example in D. A. Skoog and D. M. West, Principles of Instrumental Analysis, Holt, Rinehart, and Winston, Inc., New York, 1971, pp 678-680, and W. M. Latimer, The Oxidation States of the Elements and Their Potentials in Aqueous Solution, Prentice Hall, Englewood Cliffs, N.J., 1952. A selection of standard electrode potentials (Excex8) is shown in Table 2 below. Values are taken from Skoog and West.
If the radioactive anion to be immobilised is iodide, then preferably an oxidising agent is chosen which will not oxidise iodide ions to molecular iodine under the reaction conditions. Molecular iodine is volatile and generation of a volatile radionuclide such as 125I2 involves increased risk of exposure to radiation for the manufacturing personnel or, at least, rapid saturation of the carbon filters. However, if an oxidising agent is used which is strong enough to oxidise silver and to oxidise iodide to molecular iodine, the oxidation of the silver should occur preferentially as this is the more favourable reaction.
In one embodiment of the invention, ferricyanide is used as the oxidising agent. Under standard conditions, it is postulated that the reaction between ferricyanide and silver/silver iodide is very energetically favourable, i.e.,
Ecell=Ecathodexe2x88x92Eanode
Ecell=0.36xe2x88x92(xe2x88x920.152)
Ecell=0.512
while the possible reaction between ferricyanide and iodide would not be a spontaneous reaction under standard conditions, i.e.,
Ecell=0.36xe2x88x92(0.54)
Ecell=xe2x88x920.18
Standard conditions are given as reagent activities of 1. For example, the concentration of both ferricyanide and the reduced form, ferrocyanide, would be equal to 1 molar.
However, the impact of concentration is predicted by the Nernst equation:
Exc2xdxe2x80x2=Exc2xdxc2x0xe2x88x92(0.059/n)log(reduced form/oxidised form)xe2x80x83xe2x80x83(i)
For the oxidising half cell, this becomes:
Exc2xdxe2x80x2=Exc2xdxc2x0xe2x88x92(0.059)log([Agxc2x0]/[Ag+])xe2x80x83xe2x80x83(ii)
while the reduction half cell becomes:
E{fraction (1/2)}xe2x80x2=E{fraction (1/2)}xc2x0xe2x88x92(0.059)log([Fe(CN)64xe2x88x92]/[Fe(CN)63xe2x88x92])xe2x80x83xe2x80x83(iii)
However, in the presence of iodide, the silver oxidation is coupled to the follow-up reaction as shown here:
Oxidation: Agxc2x0xe2x86x92Ag++exe2x88x92 125Ixe2x88x92+Ag+xe2x86x92Ag125Ixe2x80x83xe2x80x83(iv)
The thermodynamics of equation (iv) are governed by the equilibrium constant, which for insoluble species is also known as the solubility product or Ksp as shown here:
Ksp=[Ag+][125Ixe2x88x92]xe2x80x83xe2x80x83(v)
Substituting equation (v) into equation (ii), the operative Nernst equation for the net reaction can be written as:
Exc2xdxe2x80x2=Exc2xdxc2x0xe2x88x92(0.059)log([Ag+]/(Ksp/[125Ixe2x88x92])
By rearranging the log term and grouping all the constants including the Ksp term into a new Exc2xdxc2x0xe2x80x2, the impact of [125Ixe2x88x92] on the overall reaction can be seen:
Exc2xdxe2x80x2={Exc2xdxc2x0+(0.059)log(Ksp)}xe2x88x920.059log[125Ixe2x88x92]xe2x80x83xe2x80x83(vi)
where {Exc2xdxc2x0+(0.059)log(Ksp)}=Exc2xdxc2x0xe2x80x2 for reaction (iv) above
Accordingly, it is postulated that in the oxidation of silver by ferricyanide, the initial conditions and those throughout the process are as follows:
This suggests that this reaction remains energetically favourable throughout the deposition of silver iodide.
The same type of process can be carried out for the possible side reaction of the oxidation of iodide to iodine. The operative Nernst equation for this half-cell is given by:
Exc2xdxc2x0=Exc2xdxe2x88x92(0.059/2)log([Ixe2x88x92]2/[I2])
These calculations suggest that in the earliest part of the reaction, there is the chance to produce 125I2 via oxidation by ferricyanide.
In a preferred embodiment of the invention, both ferricyanide and ferrocyanide are added to start the reaction to minimize the production of 125I2. Preferably, ferricyanide and ferrocyanide are added at an initial molar ratio of 10=[Fe(CN)63xe2x88x92]/[Fe(CN)64xe2x88x92]. In particular, an aqueous solution prepared from potassium ferricyanide and potassium ferrocyanide trihydrate can be made and added to the reaction vial. It is postulated that the presence of the ferrocyanide reduces the propensity for the side reaction to occur while the energetics behind the oxidation of silver to silver iodide remain very good. Those skilled in the art will recognize that similar reactions can occur in the presence of other redox couples.
The more resistant the metal is to oxidation, the stronger the oxidising agent should be. For example if the metal substrate is gold, a more powerful oxidizing agent such as permanganate may be used in the method of the invention.
The amount of oxidising agent required may be readily calculated by a skilled person depending on the amount of radioactive anion it is desired to immobilize on the metal substrate.
The amount of the radioactive anion, for example the concentration of a solution of the radioactive anion, can be chosen depending on the activity level desired in the resultant brachytherapy source. For example, substantially all of the anions present may be radioactive (i.e. xe2x80x9chotxe2x80x9d) or the radioactive anions may be diluted with non-radioactive (i.e. xe2x80x9ccoldxe2x80x9d or carrier) anions. For example, radioactive 125I-iodide may be diluted with non-radioactive 127I-iodide. Conventional brachytherapy sources for use in the treatment of prostate cancer normally have activities in the region of 0.2 to 1.5 mCi. Using the method of the invention, coated substrates with an activity of up to as high as about a Curie may be prepared. Such substrates, and radioactive sources comprising such substrates, form further features of the invention.
In order for the insoluble salt to form a stable layer which is strongly bound to the metal substrate and which does not flake or fail to adhere to the substrate, it may be necessary to also use a binding agent in the method of the invention. The binding agent preferably comprises a non-radioactive anion which also forms an insoluble salt with cations of the metal and which is different to the radioactive anion. Preferably, the salt formed by cations of the metal with the binding agent will be less insoluble, i.e. more soluble than that formed by the radioactive anion with cations of the metal or with the binding agent counter-ion. For example, if the radioactive anion is 125I-iodide, suitable binding agents include chloride or bromide ions, preferably bromide ions.
Whether or not a binding agent is required in any particular case will depend, at, least in part, on the nature of the metal of the substrate and the radioactive anion to be immobilized. Whether or not a binding agent improves the stability of the coated substrate in a particular case may be determined by routine trial and error experiments.
The applicants do not wish to be bound by any particular theory regarding the rxc3x4le played by the binding agent in the method of the invention, but it is postulated that the binding anion is preferably physically smaller than the radioactive anion such that it can fit into gaps or cracks in the coating layer to help hold the layer together. It is also possible that the binding ions take part in establishing a template on the metal surface that affords a more adhesive layer of the insoluble salt of interest.
A layer comprising both the binding agent and the radioactive anion will form on the metal surface of the substrate. Provided the reaction is carried out with sufficient mixing e.g. stirring or agitation, the radioactive anion can be homogeneously distributed throughout this layer. Silver substrates coated using the prior art two-step process of U.S. Pat. No. 4,323,055 comprise the radioactive anion in a layer of silver iodide on the surface of a silver bromide-coated silver substrate.
If the metal is silver and the radioactive ion is 125I-iodide, carrying out the method of the invention in the presence of an excess of bromide ions leads to formation of a more physically stable layer than if the bromide ions are not present. The binding agent thus enhances adherence of the Ag125I salt to the surface of the substrate. In addition, the use of bromide ions as a binding agent leads to formation of AgBr on the surface of the substrate in addition to Ag125I. The AgBr may form a coating over some or all of the Ag125I, which may help to minimize loss of radioactivity from the source due to physical handling of the coated substrate. It is postulated that the small amount of bromide ion present may serve to help establish the crystal form of the resulting AgI such that it is a more cohesive layer, and adhesive to the metal substrate. In one embodiment, the molar ratio of bromide to iodide present in the surface layer of the substrate is preferably in the range of 2.25 to 2.75 and, more preferably 2.5.
In one embodiment of the invention, I-125 may be immobilised on a silver substrate by treating the substrate with a solution or dispersion comprising I-125 ions and bromide ions and a solution of an oxidising agent, for example an aqueous solution of potassium ferricyanide. The I-125 and bromide ions may, for example, be present as an aqueous solution of Na125I and NaBr. Alternatively, the source of I-125 ions may be a dispersion of Ag125I in a suitable solution phase, for example an aqueous solution.
It is postulated that in this embodiment the following reactions occur:
Ag+Fe(CN)63xe2x88x92xe2x86x92Ag++Fe(CN)64xe2x88x92
Ag++Brxe2x88x92xe2x86x92AgBr
Ag++125Ixe2x88x92xe2x86x92Ag125I
AgBr+125Ixe2x88x92xe2x86x92Ag125I+Brxe2x88x92
If the I-125 ions are present as a dispersion of Ag125I, some of the Ag125I will gradually dissolve as the immobilisation reaction progresses thus generating I-125 iodide ions in solution. I-125 ions in solution may react directly with the oxidised silver (i.e. the Ag+ cation) to form Ag125I. Alternatively, the oxidised silver may react first with bromide ions to form AgBr, followed by ion-exchange of I-125 iodide for bromide to give Ag125I. Thus both oxidation of the silver and formation of AgBr will result in removal of I-125 from the solution phase. Substantially all of the I-125 from the solution phase is therefore immobilised on the substrate using the method of the invention. The radioactivity of the product is therefore highly dependent on the effective concentration of 125I or on the amount of 125I effectively available in the initial solution phase.
When a plurality of substrates, for example metal wires or metal coated substrates such as metal coated organic compositions including metal coated plastics or polymers, or metal coated inorganic compositions such as metal coated ceramics or glasses are treated together, the nominal amount of radioactive ion that is immobilized on the metal surface of each substrate can vary from substrate to substrate giving rise to a statistically normal distribution of an average amount of radioactivity per substrate. In a kinetically rapid reaction, the width of the statistical distribution may be controlled to some extent by increasing the volume of the reacting solution without increasing the amount of reagents, thereby effectively diluting the reagents and slowing the reaction. In addition, applicants have unexpectedly observed that the presence of a salt additive to the reaction mixture at elevated concentrations can narrow the statistical distribution of the amount of radioactivity per substrate. One possible explanation is that the increased concentration of ions in the solution decreases the solution activity coefficient of the ions of interest (i.e. the radioactive anions and/or the binding agent) and hence slows the reaction.
The concentration of salt additive can range from about 0.01 molar up to saturation levels of the salt in solution, the latter varying as a function of the salt and temperature of the solution. For example, useful salts include NaCl which has a saturation level of about 357 grams per liter in water at 0xc2x0 C. and about 391.2 grams per liter in water at 100xc2x0 C.; KCl which has a saturation level of about 347 grams per liter in water at 30xc2x0 C. and about 567 grams per liter in water at 100xc2x0 C.; CsCl which has a saturation level of about 745 grams per liter in water at 20xc2x0 C. and about 1590 grams per liter in water at 100xc2x0 C.; LiCl which has a saturation level of about 637 grams per liter in water at 0xc2x0 C. and about 1300 grams per liter in water at 95xc2x0 C.; and MgCl2 which has a saturation level of about 542.5 grams per liter in water at 20xc2x0 C. and about 727 grams per liter in water at 100xc2x0 C.; and NaNO3 which has a saturation level of about 921 grams per liter in water at 20xc2x0 C. and about 1800 grams per liter in water at 100xc2x0 C. A number of these chloride and nitrate salts have been evaluated and shown to be effective at narrowing the statistical distribution of the amount of radioactive anion, for example iodide, present from substrate to substrate.
The saturation levels of other useful highly soluble salts can be found in the Handbook of Chemistry and Physics, CRC Press, 55th Edition. Saturated solutions of added salts can be achieved when an excess of undissolved salt is present in the reaction mixture in equilibrium with dissolved salt. The optimum level of ionic strength per added salt or mixtures of added salts which promotes the narrowest distribution of radioactivity uptake per substrate can be readily found by one skilled in the art using routine experimentation. At the end of the immobilisation reaction leading to uptake of radioisotope onto the metal substrate to form a radioactive substrate, excess salts can be removed by washing the substrate with aliquots of water.
Without the added volume or the added salt, the statistical distribution of radioactivity on the individual substrates is critically dependant upon the rate of mixing. Distributions ranging from 5 to 15% (relative standard deviation) are often observed. With the addition of increased volumes, these values drop to  less than 6% (relative standard deviation). Addition of 2 M NaCl unexpectedly results in distributions  less than 3% (relative standard deviation). The additional chloride ions may not take part in the reaction, but may simply decrease the activity coefficient of the reacting ions (e.g., bromide and iodide) thereby decreasing the critical dependence on the rates of mixing of the reagents themselves. It is, however, known that silver chloride is appreciably soluble in concentrated alkali chlorides where chloro complexes are formed (see Cotton, F. A. and Wilkinson, G. in Advanced Inorganic Chemistry, Interscience Publishers, John Wiley and Sons, page 863, 1962).
If the radioactive anion is present as a dispersion or in the form of a complex or a degradable compound, there may be the advantage that thorough mixing of the substrate(s), the oxidizing agent and the source of the radioactive anion occurs before the immobilisation reaction begins, leading to a more even distribution of the radioactive anions over the substrate(s)
The use of organic salts of the radioactive anions might also be useful at slowing the reaction of iodide with the silver wires, for example pyridinium iodides may be useful as an iodide source using this approach. Adding the radioactive anion to the reaction mixture as a solid salt may also be useful as the kinetics of dissolution may slow down the exposure of the wires to solution phase anion until mixing is completed. While the solid salt is not insoluble, the change from the solid phase to the solution phase could accomplish the desired alterations in the speed of the reaction.
In the specific case of the immobilisation of radioactive iodide ions on a plurality of silver substrates using potassium ferricyanide as the oxidising agent and bromide ions as a binding agent, the addition of  greater than 1 M NaCl to the reaction mixture gives rise to a yellow green precipitate shortly after the addition of the potassium ferricyanide. This green precipitate clears as the substrates coat with a mixed silver bromide/silver iodide salt. Chemical analysis of the precipitate shows high levels of silver and trace levels of iron suggesting the nature of the precipitate to comprise silver halide. This precipitation may allow mixing to occur before the bulk of the oxidation takes place, thereby ensuring a better distribution of iodide amongst the individual substrates.
Some metal halides, for example silver halides, are light-sensitive. Among the silver halides, silver bromide is most light-sensitive, followed by silver chloride and silver iodide respectively. One disadvantage of the prior art process of U.S. Pat. No. 4,323,055 for forming silver iodide-coated silver wires was that the light-sensitivity of the silver iodide meant that the process could not be carried out under natural light, but had to be carried out under red light.
A further significant advantage of the method of the present invention is that it need not be carried out under safe lights but can be carried out under normal room (fluorescent) lights. While a colour change is observed during the washing and drying steps from lime green of the initial coated wires to a darker, olive green for dried coated wires, analyses for iodide content either by radiochemical analysis or by the spectrophotometric method described in Example 1 below do not reveal any change in amount or loss of iodide from the coated wires. Better than 99% uptake is consistently achieved under normal, fluorescent lighting. It is possible that since the iodide is distributed throughout the deposited layer using the method of the invention, the light sensitivity is not critical, whereas the previous process resulted in Iodide deposited onto the surface of an existing AgBr coating, thereby exposing the AgI directly to any light incident upon the wires. This significantly improves the ease of manufacture and handling and avoids the need for expensive dark room facilities. It also significantly improves working conditions for those personnel involved in the manufacturing process and improves the morale of the manufacturing team.